1. Field of Invention
The present invention relates in a general way to a process for the anisotropic etching of a layer of a dielectric polymer material by means of an oxygen-based plasma.
2. Description of the Related Art
At the present time, interconnects in silicon-based microelectronics are produced using aluminium to form the metal lines and silicon oxide (SiO.sub.2) as the insulation dielectric between the metal lines. However, reducing the size of integrated circuits, and therefore increasing the operating speed of the devices, requires the strategy for interconnect formation to be significantly modified, or else the operating speed of the device will be limited by the propagation time of the signals in the interconnects.
To solve this problem, it is therefore necessary to replace the aluminium with a conductive material of lower resistivity, for example copper, and the silicon oxide with a dielectric material having a lower dielectric constant.
There are a wide variety of materials having a dielectric constant lower than SiO.sub.2 (which has a dielectric constant of about 4 at the frequencies used for the envisaged applications). One particularly interesting class of materials having a low dielectric constant is that of dielectric polymer materials, in particular purely organic dielectric polymer materials. An example of a suitable polymer material is the material sold by DOW CHEMICAL under the name SILK.RTM. which has a dielectric constant of about 2.6.
In order to produce the metal interconnect lines using such dielectric polymer materials as insulation material between the lines, it is necessary firstly to produce, on a layer of the dielectric polymer material, by conventional photolithography, a hard mask, for example of silicon oxide SiO.sub.2, which defines the dimensions and shapes of the interconnect lines and holes, and then to etch the pattern of lines and holes in the layer of polymer material. Next, the etched lines and holes are filled with a material, such as copper, and then, as is conventional, the structure obtained is planarized by mechanical-chemical polishing (PMC).
The etching step is an essential operation in the process for fabricating integrated circuits and must be able to obtain etched sidewalls as straight as possible (anisotropic etching).
Furthermore, in order to understand the problems associated with the etching step, it is important to bear in mind that the copper/dielectric polymer material pair is intimately associated with the use of the so-called "damascene" process which makes it possible to define and produce lines and holes simultaneously.
This so-called "damascene" process is shown schematically in FIGS. 1a to 1d.
FIG. 1a shows a conventional "damascene" structure before the interconnect lines and holes have been etched.
As shown in FIG. 1a, this structure includes, within the layer 1 of organic polymer material, stop layers 2 and 3, for example made of oxide SiO.sub.2, which are configured and positioned so as to produce interconnect holes and lines.
After a hard mask 4 of suitable pattern has been formed, the interconnect holes 5, 6 and the interconnect line 7 are etched, as shown in FIG. 1b.
The next step is the deposition of metal (for example Cu) in the holes 5, 6 and the line 7 (FIG. 1c) and the planarization of the structure (FIG. 1d). In order to deposit this metal, any conventional method may be used, for example chemical vapour deposition (CVD).
It may be seen from FIG. 1b that the line 7 and the holes 5, 6 must be etched simultaneously, something which complicates the etching operation since the form factors involved are large. Moreover, the etching process must make it possible to achieve significant etching selectivity between the polymer material and the layer 1, for example at the intersection (S) between the line 8 and the hole 7.
The lines and holes are conventionally etched by means of an oxygen plasma which allows both high etch rates and significant and controlled directivity along the normal to the surface of the etched substrate.
The kinetics observed in plasmas are generally attributed, on the one hand, to a phenomenon of dissociation of the reactive gas at the start of the production of reactive atomic species and, on the other hand, to the ionization of the gas, which produces positive ions allowing ion bombardment normal to the surface of the substrate which is at a negative potential with respect to the potential of the plasma.
The actual operation of plasma-etching a layer of polymer material will now be described with reference to FIGS. 2a and 2b.
The action of plasma-etching a layer 1 of polymer material by means of a hard mask 2 may be decomposed into a rate of vertical etching V.sub.V in a direction normal to the layer 1 and a rate of spontaneous lateral etching V.sub.1, directed towards the etched sidewalls not subjected to the ion bombardment. As shown in FIG. 2a, the etching of the polymer material 1 has an isotropic etching profile due to the action of the lateral etching V.sub.1.
In practice, in order to obtain an anisotropic etching profile, it is necessary to increase the flux of ions bombarding the substrate compared with the flux of reactive species responsible for the spontaneous etching of the etched sidewalls.
The very high reactivity of polymers with the neutral oxygen atoms present in an oxygen plasma makes it very difficult to use pure-oxygen plasmas (when the temperature of the substrate is maintained close to 20.degree. C.). The polymer-etching profiles obtained in an oxygen plasma are not anisotropic, whatever the plasma conditions used. Etching profiles like that shown in FIG. 2a are often observed. To avoid the appearance of lateral etching, it would be desirable to use an etching chemistry, particularly an oxygen-based one, which, combined with the plasma conditions, allows the formation, as in FIG. 2b, of a passivation layer 3 on the sidewalls of the hole or of the line etched in the layer of polymer material 1. This passivation layer 3 must allow the reactions causing spontaneous lateral etching of the polymer to be blocked and therefore anisotropic etching to be obtained.
In plasma etching, a passivation layer is formed from non-volatile or low-volatility compounds which come either from the decomposition of the etching gases or from the reaction products of the etching. These low-volatility products are deposited on the sidewalls of the material from the gas phase of the plasma (when they originate from the decomposition of the etching gas) or come from the sputtering of the reactive layer by the ion bombardment of the plasma. In the latter case, the ion bombardment is conducive to the formation of low-volatility etching reaction products and sputters them onto the sidewalls of the patterns. Whatever the mechanism of formation of the low-volatility products (decomposition of the etching gas or sputtering of the reactive layer), the non-volatile products accumulate only on the unbombarded surfaces of the patterns (the etched sidewalls).
As indicated previously, in a damascene-type process, the step of etching the polymer material is followed by a step of depositing a metal such as copper or possibly aluminium. The metal is therefore in contact with the sidewalls of the polymer and in particular with the passivation layer. It is therefore paramount that the passivation layer be chemically inert with respect to the metal. In particular, corrosion reactions between the metal and the passivation layer must be avoided.
Consequently, it is necessary for the gas phase of the etching plasma to be free of elements capable of generating corrosion reactions with the metal deposited.
Furthermore, the gas phase of the plasma must also not contain elements or compounds capable of impairing the electrical properties of the etched polymer material.
Finally, it is extremely desirable for the etching step to be able to be carried out in complete safety.